Waveguide filter

ABSTRACT

Waveguide band-pass filters operated such that only evanescent H waves exist at the operating frequency of the system are terminated in a capacitive reactance which at the center frequency of the desired pass band is the conjugate of the positive imaginary characteristic impedance of the length of evanescent waveguide. The capacitive termination is the only obstacle required in the guide.

United States Patent [72] Inventor George Frederick Craven Sawbridgeworth, England [21 1 Appl. No. 643,279

[22] Filed June 2, 1967 [45] Patented Nov. 16, 1971 73] Assignee International Standard Electric Corporation New York, N.Y.

[ 32] Priority June 10, 1966 [33] Great Britain 54] WAVEGUIDE FILTER 1 Claim, 12 Drawing Figs.

52 us. Cl 333/73 w,

[51] Int.C1 H03h7/l0 [50] Field ofSearch 73 C,73 W,8l B; 331/107 References Cited UNITED STATES PATENTS 2/1970 Kawahashi 4/1955 Hall 4/1940 King 6/1951 .laynes 333/73 W 333/81 B 333/81 B 333/81 B 2,376,785 5/1945 Krasik 333/81 B 2,659,870 11/1953 Laemmel..... 333/81 B 2,427,098 9/1947 lKeizer 333/81 13 3,451,014 6/1969 Brosnahn et al.... 333/73 W 2,724,805 11/1955 Smullin 333/73 2,866,949 12/1958 Tillotson 333/1 1 3,215,955 11/1965 Thomas et al 333/7 2,231,602 2/1941 Southworth..... 333/73 2,588,103 3/1952 Fox 333/73 W 2,432,093 12/1947 Fob 333/73 W 2,816,270 12/1957 Lewis 333/73 W 2,531,447 9/1950 Lewis 333/73 W 2,623,120 12/1952 Zobel 333/73 W Primary Examiner-Herman Karl Saalbach Assistant Examiner-C. Baraff Attorneys-C. Cornell Remsen, Jr., Rayson P. Morris, Percy P. Lantzy, Philip M. Bolton and Isidore Togut ABSTRACT: Waveguide band-pass filters operated such that only evanescent H waves exist at the operating frequency of the system are terminated in a capacitive reactance which at the center frequency of the desired pass band is the conjugate of the positive imaginary characteristic impedance of the length of evanescent waveguide. The capacitive termination is the only obstacle required in the guide PATENTEDuuv 16 Ian 3 6 21 .48 3

SHEET 2 [IF 3 lnvenlor GEORGE F, CRAVf/V Agenl WAVEGUIDE FILTER BACKGROUND OF THE INVENTION This invention relates to waveguide filters.

Waveguide band-pass filters have heretofore been con structed in waveguide in which a propagating mode -usually the dominant --exists.

SUMMARY OF THE INVENTION According to the invention there is provided two lengths of waveguide, each said length of waveguide being dimensioned such that a propagating mode exists at the operating frequency of the system, and at least one H-mode filter coupling said lengths of propagating waveguide together, each of said H- mode filters including at least one section of waveguide dimensioned to have a cutofi frequency above said frequency passband, said section having a positive inductive characteristic reactance at the center frequency of said passband, and a plurality of capacitive screws located at predetermined points along the length of said waveguide, the capacitive reactance of each said screw being the conjugate of the positive imaginary characteristic reactance of said section of waveguide at the operating frequency of the system.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows two lengths of propagating waveguide interconnected by a three section band-pass filter embodying the invention,

FIG. 2 is the equivalent circuit of one section of the bandpass filter of FIG. 1,

FIG. 3 is the circuit of FIG. 2 bisected,

FIG. 4 is the characteristic of a conventional lumped circuit band-pass filter,

FIG. 5 is the characteristic of the band-pass filter according to the present invention,

FIG. 6 shows an alternative form of the band-pass filter of FIG. 1,

FIG. 7 shows another form of band-pass filter embodyingthe invention,

FIG. 8 is the approximate equivalent circuit of the bandpass filter of FIG. 7, M

FIGS. 9 and 10 show the filter section of FIG. 7 coupled as a series and a shunt stub respectively to dominant mode waveguide,

FIG. 11 is the equivalent circuit of a single section bandpass filter of FIG. 12.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1, dominant mode H waves are propagated in a length l of propagating waveguide from any suitable microwave source (not shown), such as a generator or a receiving aerial. A length 2 of propagating waveguide is interconnected with the length l by a three section band-pass filter 3 constructed entirely in evanescent waveguide, that is to say, all modes in this filter are evanescent at the operating frequency of the system.

E cosh 32,, sinh 1 0 E 'y 'y I j o sinh 5 cosh gB l I The combined matrix is then E1 cosh g zoa sinh 12, sinh E2 1 l l l (2) l 1 1 I j smh 2+]B1 cosh 2 cosh 2 I; The image impedance is then given by i v oe liu where l l Z =cosh -Z E Sll'lh 1 1 J sinh 2 +JB1 cosh 2 (3a) 1 Z,,,,- Z tank 2 the image transfer constant cos h I /2 is given by cos I: I /2 =cos h 71/2 (cos h 'yI/2 -Z,B, sin h 'yI/2) (4) B =21rf C and, therefore, these equations give the passband frequency limits in terms of the transmission line parameters (y!) and the terminating capacitance (C,). If the two passband frequencies at which (7) and (8) are satisfied are f, and f respectively.

cot

The center frequency, f,,, occurs at the geometric mean,

Therefore,

Further,

Clearly, the network of FIG. 2 is a band-pass filter, the image impedance of which is given by (3). In general, it will be desirable to match the filter at its center frequency, f,. Substituting the value of B at the center frequency (27rf,,C,=l/Z.,) in (3), the image impedance, 2 at f, is given by ia 0 tanli v 7 An obvious characteristic of the filter is that its bandwidth is a function of 1 and (in the ideal lossless case) as 'yl so then tanh 1-? coth 1 and the bandwidth (f,f,) reduces towards zero.

The behavior of the image impedance as a function of frequency is interesting. At very low frequencies 71 is extremely large and B tends to zero. Thus 2, is given by At the lower frequency band limit, f,, the denominator in (3a) becomes zero so that 2, becomes infinite. At the upper frequency limit, F}, the numerator becomes zero and, therefore, Z, is also zero. This behavior is illustrated in FIG. 5 for contrast with the characteristics of a conventional lumped-circuit band-pass filter (FIG. 4).

Returning to FIG. I, this is probably the simplest waveguide filter realizable for it consists simply of a set of capacitance screws 4, (one per section) in an appropriate size of guide. The rejection below resonance is intrinsically higher than other filters (because 'yl increases with wavelength) and the loss per sections is not excessive. The typical insertion loss for a three section filter is on the order of 0.1 db per section. As in all microwave filters unwanted passbands can occur but the usual dominant mode multiple cavity resonance effect is obviously absent. In the simplest case, when identical guide is used throughout, free transmission must be expected above the cutoff frequency for that particular guide. In order to control this effect the construction of FIG. 6 is used where the capacitive screws are replaced by capacitive ridges 5. This style of construction is, of course, identical to that of the familiar corrugated low-pass filter. It is then possible to completely suppress this passband, or alternatively, employ it as a second controllable passband in systems requiring this feature.

Networks of the type described above have useful properties as reactance networks when considered as purely single port devices. This is illustrated by the network of FIG. 3 with its output terminals open circuited. The input impedance is then given by (3a) which slightly rearranged is (COS h 14-Z0B1 sin 2 2 Z "Z T" (ZgBl cosh sin y w This network has the zero and infinity values of 2, as described above and is roughly the equivalent of the m-derived section shown in FIG. 7. The approximate equivalent circuit is given in FIG. 8. p

In this form of filter, the length of evanescent waveguide 10 is terminated by a short circuited section of propagating waveguide 11 having a length I such that tan 21rI/Ag is negative, and thus forms the required terminating capacitive reactance for the length of evanescent waveguide. This form of termination prevents energy being lost at the termination if this form of construction is used for example as a stub.

Several applications for the reactance section of FIG. 7 are now given. When coupled to conventional dominant mode guide 12, it can be used as a shunt or series stub in the usual way (13 or 14 in FIGS. (9 and 10 respectively). By using it as a shunt stub the passband appears at a lower frequency than the rejection band; as a series stub the positions of the two bands are reversed. It can, for instance, be used as the series element in a tenninating m-derived section for the evanescent waveguide filter. Alternatively, in this type of filter it can be used as part of an internal section giving high rejection at a specified frequency.

A more accurate version of the equivalent circuit of a single section filter similar to that of FIG. 1 is shown in FIG. 11, the filter 15 being shown somewhat schematically in FIG. 12 as a length of evanescent waveguide 16, with a central capacitive screw 17, between dominant mode guides 16 and I9.

The inclusion of the inductance shunt susceptances, represented by the junction with dominant mode guide, has the effect of bringing the two resonances much closer together than predicted by (3a). The 'unction susceptances, if sufficlently large can completely e iminate the resonances. Experiments with X-band guide at 4,000 MI-Iz. (junction between 2 in. X35 in. and 0.9X0.4 in. guides) failed to demonstrate this effect until the junction susceptances were tuned out with capacitive screws in shunt.

By constructing the filter to have capacitive screws at each end of the evanescent waveguide section to have the fonn of a 1r section, the capacitive screws then serve both to tune out the junction susceptances and as the terminating capacitive reactance of the filter section.

It is to be understood that the foregoing description of specific examples of this invention is made by way of example only and is not to be considered as a limitation on its scope.

I claim:

1. A waveguide system comprises:

two lengths of waveguide, each said length of waveguide being dimensioned such that a propagating mode exists at the operating frequency of the system; and

at least one I-I-mode filter coupling said lengths of propagating waveguide together, each of said I-I-mode filters ineluding:

at least one section of waveguide dimensioned to have a cut-off frequency above the frequency passband of said filter, said section having a positive inductive characteristic reactance at the center frequency of said passband; and

a capacitive screw located at a predetermined point along the length of said section, the capacitive reactance of said screw being the conjugate of the positive imaginary characteristic reactance of said section of waveguide at the operating frequency of the system for providing a frequency passband having 7 h ll cot h ll 7 equals the wavelength of the propagated energy, 1 equals the length of said section, 2,, equals the characteristic impedance of said section and C equals the capacitance of said screw, the center frequency of said passband defined by the equation j", flf2' i i i i I 

1. A waveguide system comprises: two lengths of waveguide, each said length of waveguide being dimensioned such that a propagating mode exists at the operating frequency of the system; and at least one H-mode filter coupling said lengths of propagating waveguide together, each of said H-mode filters including: at least one section of waveguide dimensioned to have a cut-off frequency above the frequency passband of said filter, said section having a positive inductive characteristic reactance at the center frequency of said passband; and a capacitive screw located at a predetermined point along the length of said section, the capacitive reactance of said screw being the conjugate of the positive imaginary characteristic reactance of said section of waveguide at the operating frequency of the system for providing a frequency passband having gamma equals the wavelength of the propagated energy, l equals the length of said section, ZO equals the characteristic impedance of said section and C1 equals the capacitance of said screw, the center frequency of said passband defined by the equation f0 f1 f2. 